1. Field of the Invention
The present invention relates generally to assembly methods and, more particularly, relates to a method for assembling a high-frequency travelling wave tube (TWT).
2. Description of the Prior Art
FIG. 1A is an exploded view of a partial disassembled travelling wave tube and FIG. 1B is a cutaway view of an assembled TWT. The figures illustrate a circuit assembly of only a portion of TWT 10. Referring to FIG. 1A. the TWT 10 includes a barrel 12 into which a helix 14 and rods 16 are inserted. The dimensions of the barrel 12, helix 14, and rods 16 are selected so that when the TWT is assembled the barrel exerts pressure on the rods 16 directly and the helix 14 indirectly via the rods 16 to maintain the position of the various parts as shown in FIG. 1B. The pressure is exerted because the combined diameter of the helix/rod assembly is greater than the inside diameter of the barrel 12. This relationship between the diameter of the workpiece to be inserted and the inside diameter is known as an interference fit.
FIG. 2 is an example of a prior art method for assembling a TWT 10. As is apparent from the above description, special provisions must be made to accomplish an interference fit because the diameter of the piece to be inserted is larger than the inside diameter of the barrel 12. In FIG. 2, a set of blades 20 is utilized to deform the barrel 12 to allow the insertion of the helix/rod assembly 17, not labelled in FIG. 2. As shown, the barrel diameter increases between the blades and the rods 16 are oriented to be inserted into these regions of increased diameter. After the helix/rod assembly 17 has been inserted into the barrel 12, the pressure from the blades 20 is released and the barrel 12 returns to approximately its original shape. Accordingly, pressure is exerted on the rods 16 and an interference fit is effected.
There are several problems associated with this prior art method of TWT assembly. First, complicated tooling is required to practice the method. The blade assembly for distorting the barrel 12 is a specialized precision tool and a special fixture is needed for preassembling the helix and rods and inserting the assembly 17 into the barrel 12. Secondly, the degree of the interference fit, i.e., the difference between twice the radius of the piece to be inserted to the inner diameter of the barrel, is limited, As a result, the pressure that may be exerted on the helix/rod assembly 17 by the barrel 12 is limited. Since rods 16 are scraped against barrel 12 to obtain a maximum interference fit without overly deforming the barrel, this method of insertion may cause cracking or breaking of the rods 16. Further, variations in part sizes within tightly specified tolerances cause relatively large variations in interference fit pressures, adversely affecting uniformity of the assembly. The performance of the device is adversely affected by these variations in pressure, particularly pressures below a threshold level. The prior art does not ensure a pressure above the threshold pressure for each assembly.
It is believed that poor contact between the rods 16 and the inside of the barrel 12 reduces heat transmission from the helix to the barrel 12 via the rods 16. Also, it is believed that cracking of the rods 16 causes pulse breakup and other problems during operation of the TWT 10. Further, in some cases, the tool used for inserting the helix/rods assembly requires the rods 16 and helix 14 to be glued to a mandrel. This glue is later removed utilizing alcohol, which tends to absorb water. During operation of the TWT 10. this water may cause contamination of the TWT cathode.
Accordingly. a method of assembling a TWT that requires less tooling, that increases the degree of the interference fit, can control and produce greater, more uniform pressure, and reduces the cracking or breaking of the rods during insertion would be of great benefit to the art.